1. Field of the Invention
The present invention relates to an air-fuel ratio feedback control system in an internal combustion engine having a single air-fuel ratio sensor downstream of a three-way reducing and oxidizing catalyst converter in an exhaust gas passage.
2. Description of the Related Art
Among known air-fuel ratio feedback control systems using air-fuel ratio sensors (O.sub.2 sensors), there exists a single air-fuel ratio sensor system, i.e., having only one air-fuel ratio sensor. Note, in this system the air-fuel ratio sensor is disposed either upstream or downstream of the catalyst converter.
In a single air-fuel ratio sensor system having an air-fuel ratio sensor upstream of the catalyst converter, the air-fuel ratio sensor is disposed in the exhaust gas passage near to a combustion chamber, i.e., near the concentration portion of an exhaust manifold. In this system, however, the output characteristics of the air-fuel ratio sensor are directly affected by a non-uniformity or non-equilibrium state of the exhaust gas. For example, when the air-fuel ratio actually indicates a rich state but oxygen is still present, the output characteristics of the air-fuel ratio sensor fluctuate. Also, in the internal combustion engine having a plurality of cylinders, the output characteristics of the air-fuel ratio sensor are also directly affected by differences in individual cylinders, and accordingly, it is impossible to detect the mean air-fuel ratio for the entire engine, and thus the accuracy of the control of the air-fuel ratio is low.
On the other hand, in a single air-fuel ratio sensor system having an air-fuel ratio sensor downstream of the catalyst converter, the non-uniformity or non-equilibrium state of the detected exhaust gas has little or no effect, and thus the mean air-fuel ratio for the engine can be detected. In this system, however, due to the capacity of the catalyst converter, the response characteristics of the air-fuel ratio sensor are lowered, and as a result, the efficiency of the catalyst converter cannot be properly exhibited, and thus the HC, CO and NOx emissions are increased.
To solve the above problems, the following method, for example, is known. Namely, the actual air-fuel ratio is adjusted by a self-oscillating term, and the mean value thereof, i.e., a coarse-adjusting term, is controlled in accordance with the output of the air-fuel ratio sensor disposed downstream of the catalyst converter.
Nevertheless, this method cannot eliminate the increase of HC, CO and NOx emissions, because a convergence error in the stoichiometric air-fuel ratio occurs due to the phase-difference between the input and output of the exhaust gas, which is caused by the low response of the air-fuel ratio sensor.
To solve the above problem, the present inventor have suggested a method of using a proportional O.sub.2 storage term and an integral O.sub.2 storage term, to forcibly shift the coarse-adjusting term when the output of the air-fuel ratio sensor disposed downstream of the catalyst converter is outside a predetermined region, in addition to the air-fuel ratio control by the coarse-adjusting term (see Japanese Unexamined Patent Application (Kokai) Ser. No. 1-66441 published on Mar. 13, 1989).
Nevertheless, the above method cannot eliminate the increase of HC, CO and NOx emissions when the catalyst converter is deteriorated, because the O.sub.2 storage effect of the catalyst converter is reduced. As a result, the proportional O.sub.2 storage term is continuously used, and accordingly, the drivability becomes poor because the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor has a large fluctuation.